1. Field of the Invention
This invention relates to the field of stereolithography, which is a technique for making solid, three-dimensional objects (or "parts") from solidifiable materials (e.g. fluid or fluid-like material such as photopolymers, sinterable powders, and bindable powders).
In recent years, stereolithography systems, such as those described in U.S. Pat. No. 4,575,330, issued Mar. 11, 1986 and entitled "Apparatus for Production of Three-Dimensional Objects by Stereolithography," have come into use. Basically, stereolithography is a method for automatically building complex three-dimensional parts by successively solidifying thin cross-sectional layers. These layers may be composed of photopolymer resin, powdered materials, or the like. Some types of powder materials are converted from a fluid-like medium to a cohesive cross-section by melting and solidification. The layers are solidified on top of each other consecutively until all of the thin layers are joined together to form a whole part. Photocurable polymers change from liquid to solid upon exposure to synergistic stimulation. Many photopolymers exist whose photospeed (rate of transformation from liquid to solid) upon irradiation with ultraviolet light (UV) is fast enough to make them practical model building materials. In a preferred system, a radiation source (e.g., an ultraviolet laser) generates a beam which is focused to a small intense spot which is moved across the liquid photopolymer surface by galvanometer or servo type mirror x-y scanners. The scanners are driven by computer generated vectors or the like. The material that is not polymerized when a part is made is still functionable and remains in the vat for use as successive parts are made. With this technology, the parts are literally grown from a vat of fluid-like material (e.g. resin or powder). Specifically, the parts are grown from a thin layer near a surface of the vat of fluid-like material. In this manner precise complex three dimensional patterns can be rapidly produced. This method of fabrication is extremely powerful for quickly reducing design ideas to physical form for making prototypes.
This technology typically utilizes a stereolithography apparatus, referred to as an "SLA," which generally includes a laser and scanner, a photopolymer vat, an elevator, and a controlling computer. The SLA is programmed to automatically make a three-dimensional part by forming it as a sequence of built-up cross-sectional layers.
Stereolithography represents an unprecedented way to quickly make complex or simple parts without tooling. Since this technology depends on using a computer to generate its cross-sectional patterns, there is a natural data link to computer aided design and manufacture (CAD/CAM). However, such systems have presented challenges relating to structural stress, shrinkage, curl and other distortions, as well as resolution, speed, accuracy and difficulties in producing certain object shapes.
2. Related Patents and Applications
The following U.S. patents, U.S. patent applications, and PCT patent application are incorporated herein by this reference as though fully set forth herein:
______________________________________ Pat. App. Status ______________________________________ U.S. 06/638,905 U.S. Pat. No. 4,575,330 U.S. 07/429,435 U.S. Pat. No. 5,130,064 U.S. 07/331,644 U.S. Pat. No. 5,184,307 U.S. 07/269,801 Abandoned U.S. 07/182,830 U.S. Pat. No. 5,059,359 U.S. 07/415,134 Abandoned U.S. 07/339,246 U.S. Pat. No. 5,104,592 U.S. 07/182,823 Abandoned U.S. 07/182,801 U.S. Pat. No. 4,999,143 U.S. 07/183,015 U.S. Pat. No. 5,015,424 U.S. 07/182,012 Abandoned U.S. 07/268,428 Abandoned U.S. 07/429,911 U.S. Pat. No. 5,182,056 U.S. 07/415,165 Abandoned U.S. 07/265,039 Abandoned U.S. 07/249,399 Abandoned PCT/US89/04096 WO 90/03255 U.S. 07/429,301 Abandoned ______________________________________
U.S. patent application Ser. No. 07/429,435, filed Oct. 30, 1989, now U.S. Pat. No. 5,130,064, describes some main features of the present invention. U.S. patent application Ser. No. 07/331,644, filed Mar. 31, 1989, now U.S. Pat. No. 5,184,307, describes in great detail the presently preferred stereolithographic apparatus, as well as various methods to form parts therewith. This application is incorporated herein by reference, including its appendices, as though fully set forth herein to facilitate handling due to its relatively lengthy disclosure. Two reference manuals, The SLA-250 User Reference Manual and The SLA-500 Reference Manual are hereby incorporated into this disclosure by reference as though fully set forth herein. These manuals accompanied U.S. patent application Ser. No. 07/429,435, filed Oct. 30, 1989, now U.S. Pat. No. 5,130,064, as Appendices B and C respectively.
U.S. Pat. No. 4,575,330 to Hull discusses stereolithography in general. It teaches complete polymerization of each cross-section in the formation of a stereolithographically-formed object.
U.S. patent application Ser. No. 07/415,134, filed Sep. 29, 1989, now abandoned, describes off-absorption-peak wavelength post curing of parts which were formed based on the primary approach to building stereolithographic parts.
U.S. patent application Ser. No. 07/339,246, filed Apr. 17, 1989, now U.S. Pat. No. 5,104,592, describes several methods of reducing curl distortion.
U.S. Pat. No. 4,999,143 describes the use of web supports to support and minimize curl in a part being formed.
U.S. Pat. No. 5,015,424 describes the use of "smalleys" to minimize curl.
U.S. patent application Ser. No. 07/429,911, filed Oct. 27, 1989, now U.S. Pat. No. 5,182,056, describes the use of multiple penetration depths in the stereolithographic process, along with the use of beam profile characteristics in combination with resin parameters to predict various cure parameters associated with the creation of stereolithographic parts. This application also describes the role of beam profile information in the creation of skin fill and discusses various multiple wavelength curing methods for reducing part distortion.
U.S. patent application Ser. Nos. 07/415,168, 07/268,428, and 07/183,012 all of which are now abandoned, disclose various methods of finishing a stereolithographic part surface to smooth out discontinuities in a post-processing step.
U.S. patent application Ser. No. 07/429,301 discusses post-processing techniques.
In the normal practice of stereolithography, objects or "parts" are built on a layer-by-layer basis, where each layer represents a thin cross-section of the part to be formed. Initial approaches to stereolithographic part building were based on the complete filling (e.g. substantial polymerization of all regions of a cross-section to a thickness at least as deep as the layer thickness) of layers. This filling was either done by the scanning of a pencil of light, by a focused or defocused pencil of light, or by flood exposure of an appropriate cross-sectional image. The pencil of light approach strictly used complete filling of cross-sections based on the scanning of adjacent overlapping vectors until the entire cross-sectional pattern was cured. These initial approaches suffered from several drawbacks, including distortion, curl, inaccurate sizing, lack of structural integrity, and lack of uniformity in down-facing surface appearance.
Later stereolithographic techniques used an internal lattice of partially cured building material ("cross-hatch" or "hatch") in place of completely filling the successive cross-sections. The internal structures primarily consisted of cross-hatch separated by untransformed building material (e.g. liquid photopolymer or the like). In this approach, the outer and inner edges of each layer are solidified by scanning what are called "boundary vectors" (also termed, "boundaries" or "border vectors" or "borders"). These vectors separate the interior solid regions of a cross-section from exterior untransformed building material. Cross-sections or portions of cross-sections that bound external regions of the part are completely filled with skin fill (termed "fill" or "skin") after being cross-hatched. The hatch insured adequate support for the "skin" as it is being created, thereby minimizing distortion.
The skin, crosshatch, and borders trap untransformed building material (e.g. liquid photopolymer) internally in the part structure and hold it in place while the part is being created. The trapped untransformed building material (e.g. liquid photopolymer) and at least partially transformed building material (e.g. at least partially cured polymer) which forms the boundaries, hatch, and skin are brought to full transformation (e.g. polymerization) in a later process known as "post curing". For additional information on post-curing, see U.S. patent application Ser. No. 07/415,134, filed Sep. 26, 1989, now abandoned.
Fairly extensive post-curing can be required when the internal cross-hatch lattice only defines discrete x-z and y-z, planes, or the like, which are separated from each other by more than the width cured by a beam, as in such cases long vertical corridors of unpolymerized material remain substantially uncured until post-processing. It is an object of the invention to provide a method of reducing or eliminating post-processing time and associated distortions while increasing structural integrity of the stereolithographically formed part.
Stereolithographic building techniques have upon occasion resulted in down-facing features having a "wafflish" appearance and texture. This appearance and texture are due to inappropriate curing techniques being used on regions of layers that contain down-facing features. When down-facing features are given both hatch and skin fill, there can be overexposure of the regions where the hatch and fill coincide. Similarly, over-exposure can occur at the points of intersection of cross-hatch vectors. In the past, it has been possible to ignore the requirement of uniform cure depth for down-facing features, since other accuracy-related errors overshadowed this effect. However, as the stereolithography art strives for and attains increasingly higher levels of accuracy, imperfections such as these can no longer be overlooked. It is an object of the invention to correct these imperfections in combination with improved building techniques.
It is also an object of the invention to obtain accurate skin thicknesses without the need of periodically building test parts and without the need of being concerned with energy distribution in the beam (beam profile). Traditionally, the methods used to estimate skin depth were only guesses that had a remote connection to actual experimental data or theoretical expectations. The actual skin thicknesses obtained by these traditional approaches were strongly dependent upon beam profile characteristics, skin vector spacing, drawing speed, and resin characteristics. However, these parameters were not coordinated to yield a particular skin thickness. For example, skin thicknesses intended to be 20 mils could easily range from 15 to 25 mils. In the past, this type of thickness range has been tolerated, but as the art of stereolithography advances, there is an increasing need for more accurate and less cumbersome methods of predicting the required exposure to obtain a desired skin thickness.